Many electronic devices are designed with auxiliary power supplies that turn on when a main power supply fails. For instance, it is desirable and, in fact, standard practice to provide an on-board battery back-up power source in computers for keeping the time and date clock circuitry running when the main power source for the computer is off so that the computer always has the current date and time available. For example, in a laptop computer, a power supply detector circuit must be provided to detect when the portable laptop computer is plugged into an AC outlet. When it is plugged into an AC outlet, power to run the computer is supplied from the AC outlet. However, when the computer is unplugged, that absence of the power from the AC input terminal of the computer must be detected so that the computer can be switched over to operate from the on-board battery power supply.
In general, the power loss detector circuits of the prior art comprise a comparator for comparing the voltage supplied by the main power supply to the voltage supplied by the auxiliary (e.g., battery) power supply. The voltage provided by the main power supply and the voltage provided by the battery are provided to the two inputs of a comparator through respective voltage dividers. The voltage dividers are ratioed so that the comparator output switches states when the main power supply drops below a predetermined threshold. For instance, in a notebook computer, the 120V AC power available from an outlet is converted to 3.3V DC which is used to power all the circuits in the computer. The battery, on the other hand, may be regulated to provide 3V of power. The resistor dividers corresponding to the main power and the auxiliary power may be ratioed such that the comparator output will switch states when the main power supply drops below 2.8V. The output of the comparator is then used as a main power supply loss indicator signal. When that signal switches state, indicating that the main power supply has dropped below 2.8V, a power supply switching circuit switches to auxiliary power.
U.S. Pat. No. 5,457,414 entitled Power Supply Loss Sensor discloses another power loss detector circuit. In the circuit disclosed in that patent, ring oscillators and other digital circuitry are used in the scheme for detecting power loss.
In both of the above-described schemes, the auxiliary or battery power supply is compared to the main power supply. Accordingly, a constant DC drain on the auxiliary power supply is needed for the operation of the power loss detector circuit. Over very long periods of time between rechargings, the battery can be completely drained.
Accordingly, it is an object of the present invention to provide an improved power loss detection method and apparatus.
It is another object of the present invention to provide a power loss detection method and apparatus which does not consume DC power from the auxiliary power supply.